Most zombie movies (of which there are dozens if not hundreds) have the monsters vulnerable to being destroyed. A single zombie is generally not much of a threat; their strength comes from their numbers.
On the opposite end you have the kaiju genre; Godzilla and his successors. These monsters are individually powerful and hard to kill. But they are vulnerable to normal attacks if enough firepower is gathered together.
Are we just going to disregard Starship Troopers?
“Kill’em all!!!”
Not a horror movie.
Real tear-jerker.
Them (another 1950’s movie)
Army of Darkness
I thought Monsters (2010) was one of the best I’ve seen in a while.
Killer Klowns from Outer Space fits all the criteria laid out in the OP.
Are we counting Tremors as horror?
Because I’ve never laughed so hard during a horror movie.
What about all of the horror movies where the “monster” is just an ordinary human? Scream comes to mind, here.
I saw this movie at a drive-in theater when I was in high school, in '82. It was clearly inspired by Alien, but was essentially an exploitation horror film.
As the title suggests, the alien impregnates a female member of the ship’s crew. It was an excuse for some nudity, but what completely broke the scene for us was that the alien’s penis was depicted as a clear plastic tube (they showed the tube, but no penetration, because it wasn’t an X-rated film), and when he impregnated the woman, green goop flowed through the tube. We couldn’t stop laughing. 
Army of Darkness, I’ll agree, fits the OP’s criteria. The two preceding films (Evil Dead) not so much, where the main adversary is an unseen, inescapable, rapidly moving camera!
Ask any AHS fan: All monsters are human!
Van Vogt apparently thought it was stolen from him. He at least got a few bucks out of it.
Van Vogt had a bit of a case for his story “Black Destroyer”, which was, AFAIK, the first case of “Monster Loose on a Spaceship”. Not to mention "Discord in Scarlet by him, the second such case (which also featured the alien implanting its eggs in one of the crew’s body). Both were combined in the “fix up” novel The Voyage of the Space Beagle.
But the plot of It! The Terror from Beyond Space is utterly different from Van Vogt’s story. So is the plot of Alien, which far more closely resembles It than van Vogt’s story.
And I’ve long suspected that Dan O’Bannon – a more movie-sensitive than literary guy – got his idea for the Alien Implanting its Young i9n the Spaceman from the Roger Corman epic Night of the Blood Beast.
Pitch Black, even though the ecology of the planet is beyond fanciful.
The very under-rated Mimic.
I sorta liked “Mimic”, but I also rather wish that they’d tried to hew closer to the original story Mimic by Donald A. Wollheim, on which it’s based. I read the story years before, and was hoping that they’d do a more faithful adaptation. I realize that movies aren’t short stories, and that the emphases are different. t was good to see some of the ideas realized onscreen, but an awful lot of the story felt like empty padding.
The story came out in 1942 originally, and has been reprinted many times. It rarely got the cover illustration, though. Here’s the one time it did:
From Dusk Til Dawn
Night of the Living Dead (I prefer the original. The ending as taken on a new meaning to me since BLM became a thing.)
The Hidden is a great B-movie about an alien criminal on the loose in Eighties L.A. Lots o’ fun: The Hidden (1987) Official Trailer - Kyle MacLachlan, Michael Nouri Alien Crime Movie HD - YouTube
I’m surprised Prometheus hasn’t been mentioned, as it’s a prequel of sorts to aliens.
I wholeheartedly recommend The Hidden, with the understanding that “B-movie” is more an economic rating than a critical one. The movie is an excellent relatively low-budget science fiction action movie. It feels as if it borrows (steals?) an awful lot from Hal Clement’s classic SF novel Needle.
Cleverly and wittily written, it features Kyle McLaughlin as a very weird FBI agent from the Pacific Northwest three years before he played a very weird FBI agent in the Pacific Northwest on Twin Peaks. I like to think that this film explains how he got that way.
The SciFi Channel (before it became Syfy) made two sequels to it that were awful.
There’s no way, though, that I would compare this film in any way to Alien.